1. Field of the Invention
The present invention relates to an optical disc and an optical disc recording/reproducing apparatus and particularly relates to an optical disc having a preliminary recorded area for reproducing, an area for so called write once which can be reproduced plural of times, and an area for recording and reproducing for many times, and a manufacturing method of the optical disc, and a recording/reproducing apparatus for the optical disc.
2. Description of the Related Art
There are some kinds of optical discs such a ROM (read only memory) disc on which an information is prerecorded, a WO (write once) disc on which an information can be recorded once and reproduced repeatedly, and a RAM (random access memory) disc which an information can repeatedly be recorded on and reproduced from.
There are many kinds of ROM discs such as a CD (compact disc), a CD-ROM, and an LD (laser disc). These are used for delivering media of mass produced softwares or information programs of such as audio, video, or computer program at a moderate price. However, a user cannot record an information on such ROM disc.
On the other hand, as well known, there are the WO disc and the RAM disc, on which an information can be recorded (these discs will be called a "RAM disc" hereafter). The RAM disc has an advantage of not only being able to reproduce an information, but also being able to record an information. On the contrary, the RAM disc is not suited for mass production, because it is difficult to produce a replica thereof.
An optical disc having merits of both of the ROM disc and the RAM disc, which has an exclusive area (ROM area) for reproducing, and another exclusive area for a function of WO or RAM (an area for a function of WO or RAM will be called a "RAM area" hereafter) is disclosed in the following prior arts; Japanese Patent Laid-Open Application No. 1-286135/1989, Japanese Patent Laid-Open Application No. 2-42652/1990, Japanese Patent Laid-Open Application No. 2-235227/1990, Japanese Patent Laid-Open Application No. 4-167238/1992, and Japanese Patent Laid-Open Application No. 4-310655/1992.
Japanese Patent Laid-Open Application No. 1-286135/1989, Japanese Patent Laid Open Application No. 2-42652/1990, Japanese Patent Laid-Open Application No. 2-235227/1990, and Japanese Patent Laid-Open Application No. 4-167238/1992 disclose an optical disc having information pits in an exclusive area for reproducing and a guide groove for guiding a pickup in a RAM area, which are formed on a same plane of the optical disc by a method such as injection molding. The RAM area may be formed on the optical disc by selectively depositing a recording material on the surface thereof or depositing the recording material on the entire surface of thereof and removing selectively the recording material by etching. Japanese Patent Laid-Open Application No. 4-310655/1992 discloses an optical disc comprised of adhered two plates, one of which is a ROM plate and another a RAM plate. To operate the optical disc of adhered two plates, an optical beam has to irradiate each plate thereof.
The prior arts mentioned before have shortages described in the following.
Adequate values of depth and width of the pit of the ROM area and those of the groove of the RAM area are essentially different each other. In general, the adequate depth of the pit of the ROM area is deeper than that of the groove of the RAM area, and the width of the pit is narrower than that of the groove. Accordingly, it is difficult to provide both of the ROM area and the RAM area on a same plane of the optical disc of high density and high quality by utilizing technologies disclosed in Japanese Patent Laid-Open Application No. 1-286135/1989, Japanese Patent Laid-Open Application No. 2-42652/1990, Japanese Patent Laid-Open Application No. 2-235227/1990, and Japanese Patent Laid-Open Application No. 4-167238/1992, for obtaining good repeatability and inter-changeability of the optical disc.
Japanese Patent Laid-Open Application No. 1-286135/1989 discloses a method of depositing a recording material by sputtering or vacuum evaporation after the pits and the groove are formed by injection moulding. In this case, an exposure condition of a laser beam to a photoresist formed on a glass disc is controlled so as to form the pits and the groove having predetermined values of depth and width. However, it is especially difficult to form the groove with a good repeatability due to a sensitivity dispersion of photoresist.
Japanese Patent Laid-Open Application No. 2-42652/1990 and Japanese Patent Laid-Open Application No. 4-167238/1992 disclose a depositing method of recording material by spin coating after the pit and the groove are formed by injection molding. However, it is difficult to control the depth and the width of the groove with good repeatability as the groove tends to be filled with the recording material in the spin coating process.
Japanese Patent Laid Open Application No. 2-235337/1990 discloses such a disc that a ROM area is formed first over a surface of an optical disc, thereafter a phase change material is deposited to cover the full surface thereof. The ROM area underneath is exposed by removing selectively the phase change material in such a manner that the area to be removed to expose the ROM area is irradiated by a weak light, such as a laser power of 1.5 mW, then removed by sputtering etching process. Another area of the phase change material is irradiated by a strong light, such as a laser power of 5 mW, so as to leave the area as the RAM area, as the area irradiated by the strong light is resistant to sputtering etching process. Such a phase change material for an optical disc is generally covered by a dielectric material. However, it is difficult to remove the phase change material adequately in the above etching process, because it is difficult for the irradiating light to distinguish the phase change material from the ROM area underneath, resulting that the ROM area to be not cleanly exposed in the etching process causing a rough surface thereof.
Japanese Patent Laid Open Application 4-310655/1992 discloses the optical disc having two plates, namely, one is the ROM area and another is the RAM area. When the optical disc recording/reproducing apparatus has only one optical pick up, turning over of the optical disc is needed for operating whole contents thereof and causes inconvenience to a user. When a plurarity of optical pickups are provided, the optical disc recording/reproducing apparatus becomes complicated in structure thereof, and expensive.
As mentioned before, when an information signal is reproduced from or recorded on the groove or the ridge, it is difficult to manufacture the optical disc of the prior art which has a good repeatability and interchangeability, but it may be possible to form a depth and a width of the groove into respective predetermined values by controlling an intensity of the laser beam of the optical pickup irradiating a photoresist on a glass disc. As for a DVD (digital video disc), which is now on a stage for standardization, a signal is recorded on both of the groove and the ridge, however, such practice is difficult to achieve by available technologies of the prior arts. A form of cross section of the groove of a DVD-RAM is to be a rectangular or a trapezoid, which is determined by a thickness of the photo resist. The DVD-RAM may have a ROM area, which is preferred to have a smaller recording density than that of the DVD-ROM having a ROM area only.